We have been using Xenopus oocytes as a model system to study the biochemical mechanisms of signal transduction pathways, especially for the functional studies of ras proteins and various BRMs. Our studies have been extended beyond oocyte maturation in order to obtain a better understanding in amphibian embryogenesis. We have established a tissue culture system in which the mesodermal cells differentiate to hemopoietic cells in early embryo of Xenopus laevis. The hemopoiesis was determined by the production of hemoglobin which was detected on the second day of the culture. In the absence of exogenous factors in tissue culture, we have shown that ventral mesodermal tissue at the late gastrula (after stage 12) has already committed itself to the differentiation for hemopoietic lineage, whereas ventral mesodermal tissue at the early gastrula (stage 10) does not differentiate to hemopoietic cells. Addition of TGF-Beta1 (but not TGF-Beta2) to the tissue culture stimulates the hemopoiesis of ventral mesodermal tissue from the early gastrula (stage 10).